Mosaic's laser-beam measurement of sinkhole reveals it's 220 feet deep, up to 152 feet wide

Mosaic, the world's largest phosphate company, now knows exactly how big the sinkhole at its Mulberry plant is, company officials said Monday. It's 152 feet across at its widest, and 220 feet deep — making it one of the deepest sinkholes in the state.

And so far, phosphate company officials say, testing has found no indication that the 215 million gallons of contaminated water that poured down the hole when it opened in August have spread through the aquifer to taint the drinking water of their neighbors. Three wells have turned up with somewhat elevated levels of pollutants, but those pollutants do not match anything from Mosaic.

Company officials hope to seal the hole before the 2017 rainy season begins next May, said Herschel Morris, Mosaic's vice president of operations. He had no estimate of how much the company is spending trying to fix the problem, which Mosaic and state officials both kept quiet about for three weeks when it first occurred.

"Mosaic is definitely putting all its resources into dealing with this," Morris said.

When the sinkhole first opened, company officials' estimates of its depth varied from 300 feet to 700 feet, so it's not as deep as they had once feared.

However, 220 feet is far from shallow. The deepest sinkhole ever measured by the Florida Geological Survey went down 250 feet, according to Florida Department of Environmental Protection records.

There is no pond water on the bottom, Morris said, but the gyp stack debris cluttering the bottom is porous enough to let water pass through it.

When the sinkhole first opened, Mosaic officials and its consultants could only see the top, which measured 45 feet across. Now they know that the bottom is much wider than that, more than 150 feet. At its narrowest, it's 25 feet across.

The phosphate company used a combination of cutting-edge technology and home-grown construction savvy to put together a map of the vase-shaped sinkhole, Morris said. Their goal: Establish a safe spot from which to shoot concrete or grout into it, then fill it up the rest of the way with gypsum, sealing it up.

Their survey was conducted with a device called LiDAR, which stands for "light detection and range." It measures the distance to a target by illuminating that target with a laser, the way sonar does with sound. To encase the LiDAR in a protective cage that could be lowered into the sinkhole, though, required the creative services of Sanchez Brothers Custom Fabrication, a Ruskin company that normally makes T-tops for boats, Morris said. The company came up with a design to house the device and keep it safe.

The cage also contained gyroscopes to keep the LiDAR level, and seven cameras to keep an eye on what the LiDAR device encountered as it was lowered into the hole, switching to infrared when the darkness became too deep. Mosaic officials also flew a drone over the hole while the cage was being lowered down, to make sure it was positioned properly, Morris said.

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Now that they know the shape of the sinkhole, he said, they can begin moving equipment into place to begin filling it, which they hope to begin working on in December.

The sinkhole at Mosaic's Mulberry plant opened up on Aug. 27, and although it allowed contaminated water to flow into the aquifer, neither Mosaic nor the DEP notified neighbors or the general public for three weeks. State law does not require public notification until the pollution shows up outside the boundaries of a landowner's property.

Gov. Rick Scott initially defended the DEP's silence, then announced the law was stupid and should be changed. Scott told the DEP to begin requiring any company or government agency that has a pollution spill to report the incident to the public, through the media, within 24 hours.

Mosaic was the first company to make such a report, announcing an accident at its Bartow fertilizer manufacturing plant involving a spill of 97,000 gallons of phosphoric acid solution. Mosaic said that spill had also been contained.

Over the weekend, a second such spill occurred, this time at its Plant City facility, and involved 50,000 gallons of phosphoric acid. Mosaic officials said they had contained the spill and it did not spread.

Contact Craig Pittman at craig@tampabay.com. Follow @craigtimes.